Threaded pipe connector

ABSTRACT

A riser connector uses a box and upset pin connector threadform design having S-shaped load flanks that provide a variable load angles depending on the radial position along the engaged teeth. This design promotes a load path that changes with position along the axial length of the engaged thread, as well as increasing or decreasing external loads on the connector. The connector also varies radial distention of the pin from the box in a manner that is different from one thread to the next. The threadform uses a combination of thread cuts that vary in shape, pitch, and/or thread cone angle to provide load and stab flanks, as well as a load path, that vary from one axial end of the engaged thread to the other.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates in general to threaded pipe connectorsand, in particular, to a pipe connector designed for high fatigue suchas offshore well riser connections.

2. Description of the Related Art

In some types of offshore oil and gas production, risers extend from thesea floor to a floating platform for processing and transferring thewell fluid to a pipe line. Production risers may be thousands of feet inlength and may extend in a long catenary curve from the platform to thesubsea well assembly. These risers are subject to tensile loads, bendingloads and fatigue due to current and waves. These risers may be in placefor years, and a failure can be very expensive to repair.

Generally, there are two types of production riser connections that havebeen used to produce oil and gas from a subsea wellhead to a surfaceproduction tree on a floating offshore platform. Both types utilize afemale member having an internal frustoconical circumferential surfaceprovided with a thread, and a male member having an externalcorrespondingly frustoconical circumferential surface and provided withan interfacing thread for engagement with the female member.

In the first type of production riser connection, a single thread isformed in a relatively thick walled, forged member for both the male andfemale members. These forged members are welded to sections of pipe. Thethick wall allows for such features as a highly tapered threads, threadrelief grooves, and final cross-sectional areas much greater than thepin. These features can greatly enhance both the static and fatiguestrength of the connection. This type of connection also easilyaccommodates added features such as metal to metal seals and stabguides, both internal and external to the threaded surface. Thesefeatures greatly enhance pressure integrity and operationalcharacteristics, respectively.

The second type of commonly used production riser connector is referredto as a threaded and coupled connection. In this connection, the femalemember is threaded in each end of a short coupling sleeve made fromeither a thick-walled pipe or a forging. The male members typicallyconsist of simply threading the ends of the pipe itself. This type ofconnection is lighter and less expensive than the one described above.It also eliminates the need for a weld between the connector and thepipe, which eliminates the restrictions on strength and fatigue that isassociated with the weld.

Riser connectors of both types described above have a number ofdisadvantages. For the welded-on connection type, the connection isgenerally heavy and costly. It also must be welded onto the main pipebody, and therefore becomes limited by the weld itself. The welds arecompatible with pipes of limited yield strengths. In addition, thefatigue life of the weld is substantially inferior, in most cases, tothat of the connection itself. Therefore this type of connection islimited in both structural and fatigue strength by the pipe weld.

For the threaded and coupled connection, since the entire connectionmust be formed on the limited cross-section of the pipe, there aresignificant limits on what can be utilized for the same features thatallow weld-on connectors to achieve high levels of performance. Pastpractices have also utilized thread and seal configurations that weredeveloped for casing applications, where seal integrity from internalpressure and static strength were the main objectives. While connectionsof this type typically only have slightly limited structural strength,they have significantly reduced fatigue strength. They are also somewhatcompromised in the ability to achieve reliable metal-to-metal sealsexternal to the threaded section as well as achieve an effective stabguide in this same location. Thus, an improved threadform for highfatigue threaded and coupled connections would be desirable.

SUMMARY OF THE INVENTION

The connection of the present invention has threadforms in the box andon the pin that have load flanks that engage each other at differentpoints along the length of the threadforms when fully made up. Thevarying engagement provides a load angle that varies along the length ofthe threadforms. The load flanks engage each other with varying amountsof interference, or radial distension, along the lengths of thethreadforms when fully made up, so that when a tensile load is applied,the resultant sharing of the load throughout the threadforms is fairlyuniform. Preferably, the load flanks in the central portion of thethreadforms have greater interference when made up than the load flanksin the upper and lower portions. Also, in the preferred embodiment, thestab flanks of some of the threads in the upper portion and the lowerportion of the threadforms engage each other with interference when theconnector is fully made up.

In one embodiment, the box has an internal torque shoulder that isengaged with a torque shoulder formed on the pin. An internal metal sealmember extends from the pin torque shoulder and engages a recess formedin the bore of the box. The connector may also have an externalmetal-to-metal seal comprising a seal member that protrudes from theupper end of the box and engages an outer diameter portion of the pin.Preferably, the box seal member has a recessed seal rib that isprotected from damage during insertion of the pin into the box.

The foregoing and other objects and advantages of the present inventionwill be apparent to those skilled in the art, in view of the followingdetailed description of the present invention, taken in conjunction withthe appended claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features and advantages of theinvention, as well as others which will become apparent are attained andcan be understood in more detail, more particular description of theinvention briefly summarized above may be had by reference to theembodiment thereof which is illustrated in the appended drawings, whichdrawings form a part of this specification. It is to be noted, however,that the drawings illustrate only an embodiment of the invention andtherefore are not to be considered limiting of its scope as theinvention may admit to other equally effective embodiments.

FIG. 1 is a sectional side view of one embodiment of a box and pinconnection constructed in accordance with the present invention;

FIGS. 2A and 2B comprise an enlarged sectional side view of an interfaceof the connection of FIG. 1 and is constructed in accordance with thepresent invention;

FIG. 3 is a further enlarged sectional side view of the threadform at apoint illustrated by the circled dotted line in FIG. 2A near the upperend of the threadform;

FIG. 4 is a further enlarged sectional side view of the threadform at apoint illustrated by the circled dotted line in FIG. 2A near a center ofthe threadform;

FIG. 5 is a further enlarged sectional side view of the threadform at apoint illustrated by the circled dotted line in FIG. 2B at a lowerportion of the threadform;

FIG. 6 is a still further enlarged sectional side view of a portion ofthe threadform as shown in FIG. 4;

FIG. 7 is a graph of a prior art example of a thread load distributionalong the length of the engaged thread while undergoing a tensile load;

FIG. 8 is an idealized graph of a thread load distribution of aconnection in accordance with the invention along the length of theengaged thread after full makeup, but not under a tensile load;

FIG. 9 is a an idealized graph of the connection of FIG. 8 whileundergoing a tensile load;

FIG. 10 is an enlarged sectional view of an external metal seal on thebox constructed in accordance with the present invention, and shown withthe pin being inserted into the box;

FIG. 11 is a further enlarged sectional view of the external metal sealon the box of FIG. 10, with certain features exaggerated for clarity;and

FIG. 12 is a schematic, reduced, sectional view diagram of a pinconstructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1-9, one embodiment of a pipe connector in accordancewith this invention is illustrated as a threaded connection 21.Connection 21 may be used for a variety of purposes, but is particularlysuitable for subsea oil and gas production with top tension and catenaryrisers. In this example, connection 21 comprises a coupling sleeve 22having two box or female members 23, each for engaging a pin or malemember 25. In this embodiment, pin 25 is shown integrally formed on anupset or enlarged ends of a pipe 24. The boxes 23 of coupling sleeve 22join two pins 25 of two pipes 24. For some applications, each pipe 24could have a pin 25 welded to or formed on one end and a box 23 weldedto or formed on the opposite end. Pin 25 and box 23 have a commonlongitudinal axis 26 when connected.

Referring to FIGS. 2A and 2B, box 23 has an internal threadform 27 (FIG.2A, 2B) that is tapered, having a larger inner diameter at toward itsrim or upper end and a smaller diameter at the lower end toward the noseof pin 25. The terms “upper” and “lower” are used for convenience onlybecause the boxes 23 in coupling sleeve 22 are inverted from each other.“Upper” is used to mean toward the rim of box 23, and “lower” is used tomean toward the nose of pin 25. Each thread of box threadform 27 has astab flank 29 (FIGS. 3-6) and an opposite facing load flank 31. For aparticular thread, box stab flank 29 faces toward the rim or upper endof box 23 and load flank 31 faces the opposite direction.

The pin 25 has an external threadform 33 (FIG. 2A, 2B) that isthreadingly engaged with the internal threadform 27 of the box 23. Eachthread of pin 25 has a stab flank 35 (FIGS. 3-6) that faces opposite boxstab flanks 29, and a load flank 37 that faces opposite box load flanks31. When pin 25 inserts or stabs into box 23, prior to rotation, some ofthe stab flanks 29, 35 will engage each other. Rotation to full make-upcauses the load flanks 31, 37 to engage each other, and in thisembodiment, some of the stab flanks 29, 35 will engage each other. Forconvenience, the engaged threads of threadforms 27, 33 closest to therim of box 23 will sometimes be referred to as uppermost threads 28. Thelowermost engaged threads of threadforms 27, 33 will sometimes bereferred to as lowermost threads 30 because they are the closest to thenose of pin 25.

The design of connection 21 has many unique features that are readilydistinguished from the prior art. These features and embodiments may beutilized individually on the connection, or in any combination. Forexample, in one embodiment, at least some of the load flanks 31, 37 ofboth the box 23 and the pin 25 include an S-shaped cross-sectionalprofile that provides load angles that vary along the engaged thread toform a high fatigue coupled connection. This design promotes a load paththat changes with position along the engaged thread, and with increasingor decreasing external loads on the connection 21. This design featurealso varies radial distention or deflection of the pin 25 from the box23 across an axial length of the threadforms 27, 33.

FIG. 6 is a enlarged view of FIG. 4, which is one of the threads of box23 and pin 25 at a generally central area between uppermost andlowermost threads 28, 30 (FIG. 2A, 2B). This engaged thread is sometimesreferred to as the demarcation thread 36. Each of the threads of pinthreadform 33 has a curved concave root 38 and a crest 39. In thisembodiment crests 39 are cylindrical and concentric about the axis ofpin 25, but this shape could vary. Crests 39 are at different distancesfrom axis 26. A taper line 40 intersects a corner of each crest 39, line40 being at a small angle relative to axis 26.

Furthermore, pin crests 39 have different radial dimensions or “heights”measured from the adjacent roots 38. As shown in FIG. 2A, an uppersection of pin threadform 33 has pin crests 39 with reduced heights,beginning approximately with the demarcation thread 36 of FIGS. 4 and 6.In this example, the pin crests 39 of these upper threads graduallyreduce in height from one thread to the next, with the shortest being atuppermost thread 28.

Each thread of box threadform 27 has a curved concave root 41 and a boxcrest 43. In this example, crests 43 are cylindrical, but some are atdifference distances from axis 26. A box taper line 45 intersects acorner of each box crest 43 and is at an angle relative to axis 26. Thelower portion of box threadform 27 may have box crests 43 that areapproximately the same distance from axis 26. Beginning approximatelythe thread below the demarcation thread 36 shown in FIGS. 4 and 6, theremaining box crests 43 are along box taper line 45.

Box crests 43 also have different heights or radial dimensions, relativeto box roots 41. As shown in FIG. 2B, a lower section of box threadform27 has box crests 43 with reduced heights ending approximately with thedemarcation thread 36 shown in FIGS. 4 and 6. The box threadform 27 fromthe demarcation thread 36 shown in FIGS. 4 and 6 upward may have crests43 of generally uniform heights.

Referring FIGS. 4 and 6, in the central portion of pin threadform 33,each thread has a pin load flank 37 with two separate sections 37 a, 37b formed at different angles. Pin load flank portion 37 a is radiallyfarther from axis 26 than pin load flank portion 37 b. Outer load flankportion 37 a joins pin crest 39, and inner load flank portion 37 b joinspin root 38. Both load flank portions 37 a, 37 b are curved slightly inthis example, rather than being flat facets, although flat facets wouldbe feasible for some applications. A line normal to a midpoint of outerload flank portion 37 a would incline less relative to the longitudinalaxis 26 than to a midpoint of inner load flank portion 37 b.

A transition area 37 c is located between and joins inner and outer pinload flank portions 37 a, 37 b. Tangent lines 42 a and 42 b to any partof load flank portions 37 a and 37 b, respectively, would intersect axis26 at a positive angle. Tangent line 42 c of transition area 37 cchanges from positive, where it joins outer flank portion 37 a, toperpendicular (not shown) to axis 26, to negative (shown in FIG. 6), andto back again to positive, where transition area 37 c joins inner loadflank portion 37 b. The junction of outer load flank portion 37 a withtransition area 37 c is at approximately the same axial position aswhere transition area 37 c joins inner load flank portion 37 b.Transition area 37 c thus has an S-shaped contour, and because of thecurvature of outer and inner load flank portions 37 a, 37 b, creates anS-shaped contour for the entire pin load flank 37.

Because of the truncation or reduced heights of some of the pin crests39, not all of the pin threads have the same size and shape of loadflank 37. In the upper portion of pin threadform 33, beginningapproximately one thread above the demarcation thread 36 shown in FIGS.4 and 6, the outer load flanks 37 a shorten and in this examplecompletely disappear for two or three threads, including uppermostthread 28. The uppermost two or three pin threads has only an inner loadflank portion 37 b. The outer pin load flank portions 37 a graduallydecrease in size in an upward direction from approximately the firstthread above the demarcation thread 36 shown in FIGS. 4 and 6.

The demarcation thread 36 of box 23 in the central portion exemplifiedby FIGS. 4 and 6, has a mating outer load flank portion 31 a and aninner load flank portion 31 b that engage pin load flank portions 37 aand 37 b, respectively, in thread interference once fully made up. Thatis, some radial deflection or distension of the metal of load flankportions 31 a, 31 b and 37 a and 37 b occurs. A box transition area 31 cjoins inner and outer load flank portions 31 a, 31 b together.Transition area 31 c has the same contour as pin transition area 37 c,but transition areas 31 c and 37 c are spaced apart when the connectionis fully made up, creating a gap, as shown in FIGS. 4 and 6. Tangentlines to box transition area 31 c also intersect axis 26 at positive andnegative angles.

Some of the box threads in this example do not have box load flank innerportions 31 b. Because of the truncation of the heights of box crests 43in the lower portion of box threadform 27, three or four of thelowermost box threads have only outer load flank portions 31 a. Theouter box load flank portions 31 a gradually increase in size in anupward direction from box lowermost thread 30 to a point well below thedemarcation thread 36 of FIGS. 4 and 6.

When the connection is fully made up, pin load flank portions 37 a, 37 bengage box load flank portions 31 a, 31 b, respectively in the centralportion, as illustrated by FIGS. 4 and 6. In the upper portion, asillustrated in FIG. 3, some of the threads will have only the inner loadflank portions 31 b, 37 b engaging or even none at all. In the lowerportion, illustrated by FIG. 5, some of the threads will have only theouter load flank portions 31 a, 37 a engaging each other or none at all.Furthermore, even though both pin load flank portions 37 a, 37 b and boxload flank portions 31 a, 31 b engage each other in the central area,the engagement differs within the central area, depending on how muchdeflection occurs between the threads of pin 25 and box 23.

The load flank portions 31 a, 31 b, 37 a, 37 b engage each other atdifferent points along the lengths of threadforms 27, 33 to vary theresultant force angles, illustrated by the lines F3, F4 and F5 of FIGS.2A and 2B. Upper contact force F3 is the vector result of the contactforces at inner load flanks 31 b, 37 b at and near uppermost thread 28when pin 25 and box 23 are fully made up and a tensile force applied.There is no pin outer load flank portion 37 a at and near the uppermostthread 28, as shown in FIG. 3, so box outer load flank portion 31 a inthis upper portion is not in engagement with any part of pin threadform33. Conversely, at and near the lowermost thread 30, resultant force F5is the vector result of the contact forces at outer load flank portions31 a, 37 a at full make up and under tensile load. There is no box innerload flank portion 31 b at and near the lowermost thread 30, as shown inFIG. 5, so pin inner load flank portion 37 b in this lower portion isnot in engagement with any part of box threadform 27. In the centralportion, the angle of contact force F4 under tensile load is vectorresultant of both the outer load flank portions 31 a, 37 a and innerload flank portions 31 b, 37 b.

Referring still to FIGS. 2A and 2B, upper load angle F3 is at a greaterangle to axis 26 (FIG. 1) than central load angle F4, and central loadangle F4 is at a greater angle to axis 26 than lower load angle F5. Theengagement between load flanks 31, 37 at full make-up and under tensileload has a lesser radial component in the lower portion (FIG. 5) than inthe central portion (FIG. 4), and the central portion has a lesserradial component than the upper portion (FIG. 3). In the lower portion,the more axially oriented outer load flank portions 31 a, 37 a providemost or all of the contact force. In the upper portion, the moreradially oriented inner load flank portions 31 b, 37 b provide most orall of the force. There is no bright line of demarcation between thelower, central and upper portions, as this will be a matter of designchoice.

In addition to the difference in load paths F3, F4 and F5, the amount ofthread interference between the various outer and inner load flankportions 31 a, 37 a and 31 b, 37 b varies. Thread interference is usedherein to designate deflection that occurs between pin and boxthreadforms 33, 27 when fully made up but before any axial preload.Thread interference causes distension of the load flank portions 31 a,37 a and 31 b, 37 b when fully made up, creating a preload force. InFIGS. 3-5, outlines of load flank portions 31 a, 37 a and 31 b, 37 b aresuperimposed over each other in the areas of thread interference. Agreater overlap between the superimposed outlines indicates a greateramount of thread interference.

In the central portion, as illustrated by FIG. 4, both outer and innerload flank portions 31 a, 37 a and 31 b, 37 b engage each other inthread interference. The amount of load flank interference decreasesfrom demarcation thread 36 in an upward direction. The amount of loadflank interference also decreases from demarcation thread 36 downward.At the uppermost thread 28, as shown in FIG. 3, there is no threadinterference between the inner load flank portions 31 b, 37 b. At thelowermost thread 30, illustrated by FIG. 5, there is no threadinterference between the outer load flank portions 31 a, 37 a

Pin stab flank 35 of the central area demarcation thread 36 shown inFIGS. 4 and 6 has two conical facets 35 a, 35 b in the example shown.Facets 35 a, 35 b join each other, with facet 35 a being farther outwardfrom axis 26 than facet 35 b and at a lesser angle relative to axis 26.Similarly stab flank 29 of the box demarcation thread 36 shown in FIGS.4 and 6 has two facets 29 a, 29 b that are at the same angles and matewith pin facets 35 a, 35 b. In the example shown, when made up butbefore axial preload, outer stab flank facets 29 a, 35 a are spacedapart from each other by a gap, but inner stab flank facets 29 b, 35 bmay contact each other at demarcation thread 36.

In the upper portion of pin threadform 33, as illustrated by FIG. 3, thereduced height of pin crests 33 causes pin outer stab facets 35 a togradually disappear in an upward direction. The box threads in the upperportion of threadform 27 have both inner and outer stab flank facets 29a, 29 b. The engagement of stab flanks 29, 35 in the upper portiondecreases in an upward direction, with only stab flank inner portions 29b, 35 b engaging each other at uppermost thread 28.

In the lower portion of box threadform 33, as illustrated by FIG. 5, thereduced heights of box crests 43 causes box inner stab flank portions 29b to become smaller and gradually disappear. The engagement of stabflanks 29, 35 in the lower portion thus decreases in a downwarddirection, with only stab flank outer portions 29 a, 35 a engaging eachother at lowermost thread 30 at full make up and before any axialpreload.

In this example, a certain amount of thread interference also existsbetween stab flanks 29, 35 of some of the threads at full make up of theconnection and before any axial preload. The amount of stab flank 29, 35interference increases gradually in an upward direction, beginning a fewthreads above demarcation thread 36 shown in FIGS. 4 and 6. FIG. 3 showsa significant amount of thread interference between inner stab flankportions 29 b, 35 b of uppermost thread 28. The amount of stab flank 29,35 interference gradually increases in a downward direction, beginning afew threads below demarcation thread 36 shown in FIGS. 4 and 6.Lowermost thread 30, exemplified by FIG. 5, shows a significant amountof thread interference between stab flank outer portions 29 a, 35 a atfull make up and before any axial preload. In this example, there are nothreads that engage each other with both thread interference on theirstab flanks 29, 35 and load flanks 31, 37, although such could occur.

Referring to FIG. 2B, box 23 has an internal torque shoulder 47 in itsbore 55 below lowermost thread 30. Pin 25 has an external torqueshoulder 49 that contacts torque shoulder 47. Sufficient tightening ofconnection 21 after contact of shoulders 47, 49 causes deflection ofshoulders 47, 49, creating an axial preload force. Torque shoulders 47,49 may be flat or slightly conical as shown in FIG. 2B. The threadinterference between load flanks 31, 37 and stab flanks 29, 35 shown inFIGS. 3-6 occurs before any axial preload. Axial preload causes thecontact forces at the load flanks 31, 37 to increase and the contactforces at the stab flanks 29, 35 to decrease. Some of the stab flanks29, 35 in the upper and lower portions of threadforms 27, 33 will shiftfrom stab flank contact to load flank contact when sufficient axialpreload is applied.

The reason for the change in configuration of the threads along thethreadform lengths is illustrated in FIGS. 7-9. FIG. 7 illustrates atensile load being applied to a threadform without considering anypreload force due to thread interference. Ideally, if a 16,000 poundtensile force is applied and the connection has 16 threads, each threadwould experience 1000 pounds of force. However, the tension does notuniformly pass through the threads. Even if the connection has axialtorque shoulders that enable a preload, the highest forces will be atthe opposite ends of the threadforms. The net force between threadsmeasured at any point along the length of the threadform will graduallydrop in a central area. Thus the central portions of threadforms ingeneral carry the least portion of the tensile load, and the upper andlower portions the greater portion of the tensile load.

FIG. 8 illustrates in idealized form contact forces between threads thatexist along the threadforms 27, 33 in accordance with this inventionwhen fully made up, but without any axial preload due to engagement oftorque shoulders 47, 49 (FIG. 2B) and without being pulled in tensionfrom external loads. The threads in the positive area of the graph ofFIG. 8 have net load flank 31, 37 preload forces, while the threads inthe negative areas of the graph illustrate net stab flank preloadforces. The preload of the stab flanks 29, 35 results from the inwardmovement of pin 25 into box 23 during make up being resisted by theinterference of stab flanks 29, 35. This resistance tends to cause pin25 to be pushed out of box 23, but this outward movement is resisted byload flanks 31, 37, thus deflecting and preloading them. The preloadforces due to interference between stab flanks 29, 35 are opposite indirection to the preload forces due to interference between load flanks31, 37.

When fully made up, torque shoulders 47, 49 (FIG. 2) will be tightenedto a desired preload force. Even without torque shoulders 47, 49,because of the various thread interferences, preload forces as in FIG. 8will exist when fully made up. The axial preload force caused by theaxial deflection of torque shoulders 47, 49 (FIG. 2) does notsubstantially change the shape of the graph of FIG. 8, rather it mainlyshifts it upward and somewhat flattens it out. Increasing the axialpreload by deflecting torque shoulders 47, 49 decreases the amount ofthread preload of stab flanks 29, 35 and increases the amount of threadpreload of load flanks 31, 37.

FIG. 9 is an idealized example of the contact forces occurring on eachthread when an external tensile load is applied to the pin having thethreadform graph of FIG. 8. The demarcation thread 36, illustrated inFIGS. 4 and 6, experiences the maximum load flank preload according toFIG. 8. As an external tensile load is applied, the load distributionamong load flanks of the threads takes the shape of FIG. 7. When addedto the existing load distribution shown in FIG. 8 from internal preload,the distribution of load on thread load flanks 31, 37 takes the shape ofFIG. 9. The demarcation thread 36 of FIGS. 4 and 6 will share a net loadportion of the tensile load applied to pin 25 equal to the preload atthe peak of the graph of FIG. 8 plus the external tensile load thatexists at that point.

The uppermost and lowermost threads 28, 30, and those near them have anet preload force due to stab flank 29, 35 interference, even afteraxial preload. When pin 25 is under a tensile load, the tensile loadlifts the interfering pin stab flanks 35 from the box stab flanks 29 andmoves the load flanks 31, 37 of those threads into contact with eachother. The resultant contact force on threads 28, 30 equals the tensileload being applied plus the stab flank preload, which is negative, thusit subtracts. If sufficient tensile load is applied, the net thread loadon the uppermost and lowermost threads 28, 30 becomes positive becausethe external load initiates contact of their load flanks 31, 37. Thecontact forces on the uppermost and lowermost threads 28, 30 is lessthan the high level that would exist in the prior art shown by FIG. 3because of the subtraction of the stab flank preload forces. Typically,the net load on the threads between lowermost thread 30 and demarcationthread 36 is somewhat less than the net loads at lowermost thread 30 anddemarcation thread 36. Similarly, the net load between demarcationthread 36 and uppermost thread 28 is typically less.

Still another design feature of the present invention deals with aninternal seal 51 (FIG. 2B). Seal 51 is a metal-to-metal seal in pin 25that engages a recess 53 in bore 55 of box 23. Seal 51 has a smallercross-section thickness than pin 25 at torque shoulder 49 and dependsfrom torque shoulder 49. The free end of seal 51 does not contact anyshoulder within box bore 55. Thus, the internal seal 51 isnon-shouldering and metal-to-metal between the box 23 and the pin 25.The free end of seal 51 defines the lowermost end or nose of pin 25.

Yet another design feature of the present invention deals with anexternal seal 61 (FIGS. 2A, 10 and 11) on box 23. Seal member 61 hassimilarities to internal seal 51. A shoulder 65 is formed near the upperend of box 23. Seal member 61 has a nearly flush inner diameter withbore 55 of box 23 and an outer diameter less than box 23 at shoulder 65.Seal member 61 thus has a thinner cross-section than box 23 at shoulder65, extends axially past shoulder 65, and defines the rim of box 23. Thefree end of seal member 61 is the uppermost point of box 23. Seal member61 engages an outer diameter portion of pin 25 to form a metal-to-metalseal. A relief groove 63 may exist at the junction of shoulder 65 withseal member 61 to increase the flexibility of seal member 61. Shoulder65 is shown flat, but it could be conical or tapered.

Referring now to FIGS. 10 and 11, in the preferred embodiment, sealmember 61 has a rounded, annular sealing rib 67 on its inner diameter.Sealing rib 67 is convex and protrudes inwardly toward axis 26 (FIG. 1).A concave, curved recess 66 is joins and extends upward from sealing rib67, and a concave, curved recess 68 joins and extends downward belowsealing rib 67. Recesses 66, 68 have depths sufficient so that sealingrib 67 is recessed from an imaginary conical surface 69 extending in astraight line from the lower edge of lower recess 66 to the upper edgeof upper recess 68. The minimum inner diameter of sealing rib 67 islarger than the diameter of imaginary conical 68 at the same axial pointto prevent threadform 33 (FIG. 2A) of pin 25 from striking and damagingsealing rib 67 during insertion of pin 25 into box 23.

Pin 25 has a tapered sealing surface 73 on its outer diameter that isengaged in metal to metal sealing engagement with box sealing rib 67when box 23 and pin 25 are made up. FIG. 10 shows pin 25 partiallyinserted into box 23. Pin sealing surface 73 may be slightly rounded andis located between cylindrical surfaces 75 and 77 on the outer diameterof pin 25.

Another feature of the present invention is schematically shown in FIG.12. Pin 25 is upset at the axial end, including an axial upset length“L” and a radial upset dimension “e”, such that an aspect ratio L/e≧30.In one embodiment, the pin 25 is upset with the radial dimension ofapproximately 0.25 inches. Also, preferably the length of thethreadforms 27, 33 is no more than two-thirds the length L.

The present invention has several advantages, including high fatigueresistance and low torque for make-up. The strength of the connectionexceeds that of the components. The stress amplification factor (SAF) islower than prior art designs. The box design features include externaland internal primary metal seals and a load shoulder to provide amajority of the preload. The thread design features include a threadformoptimized for strength and fatigue, and threads that assist with allloading conditions while performing better under bending loads. Thedistribution of loading between the threads along the axial length ofthe thread during preload and subsequently applied axial loads can betailored for many different applications. This design also providescompressive preload, bending resistance, or socket action in a specificregion of the thread, although not necessarily the entire thread. Theconnector also has improved static strength, fatigue life, andfunctional make-up of the connection.

The present invention is well suited for many applications including,for example, 9.75 to 16 inch OD pin with a wall thickness in the rangeof 0.375 inches to over 1 inch. The pins only require a slight upset(approximately 0.25 inches), and have no welds. In one embodiment,approximately 14 to 16 threads are used on approximately a 2° taper.This design may be used in many different applications, such as multipleflow line applications like production risers, export risers, importrisers, steel catenary risers (SCR), etc.

While the invention has been shown or described in only some of itsforms, it should be apparent to those skilled in the art that it is notso limited, but is susceptible to various changes without departing fromthe scope of the invention.

1. A pipe connector, comprising: a box having internal threads; a pinhaving external threads engaged with the internal threads of the box,the threads of the pin and box having stab flanks and load flanks; andthe load flanks of a central portion of the threads of the pin and boxengaging each other with greater interference than in upper and lowerportions of the threads.
 2. The connector according to claim 1, whereinthe engagement of the load flanks of the central portion of the threadsof the pin and box creates load paths with angles relative to an axis ofthe connector that differ from load paths created by the engagement ofthe load flanks of the upper and lower portions of the pin and boxthreads.
 3. The connector according to claim 1, wherein the engagementof the load flanks of the threads of the pin and box creates load pathswith angles relative to an axis of the connector that change graduallyfrom one end of the threads to the other end of the threads of the pinand box.
 4. The connector according to claim 1, wherein: the load flanksin the lower portion, central portion, and upper portion of the threadscreate load paths with angles relative to an axis of the connector; andthe angles of the load paths in the lower portion of the threads areless than the angles of the load paths in the central portion, and theangles in the central portion are less than the angles in the upperportion.
 5. The connector according to claim 1, wherein: theinterference of the load flanks of the upper portion of the threads ofthe box and pin decreases in an upward direction; and the interferenceof the load flanks of the lower portion of the threads of the box andpin decreases in a downward direction.
 6. The connector according toclaim 1, wherein: the stab flanks of the upper portion of the threads ofthe box and pin engage each other in interference in an increasingamount in an upward direction; and the stab flanks of the lower portionof the threads of the box and pin engage each other in interference in aincreasing amount in a downward direction.
 7. The connector according toclaim 1, wherein: each of the load flanks of the central portion of thethreads of the pin comprises an outer portion and an inner portion thatare joined to each other by a transition area that extends slightlydownward from its junction with the outer portion to its junction withthe inner portion.
 8. The connector according to claim 1, wherein: eachof the load flanks of the central portion of the threads of the pincomprises a curved, convex outer portion and a curved, concave innerportion that are joined to each other by a transition area; a linetangent to a junction of the outer portion with the transition areaextends in an upward and inward direction to an intersection with anaxis of the connector; and a line tangent to a junction of the innerportion with the transition area extends in a downward and inwarddirection to an intersection with the axis of the connector.
 9. Theconnector according to claim 1, wherein in the upper portion of thethreads: at least some of the load flanks of the threads of the box andpin are non interfering with each other and at least some of the stabflanks of the threads of the box and pin interferingly engage each otherwhen the connector is fully made up.
 10. The connector according toclaim 1, wherein in the lower portion of the threads: at least some ofthe load flanks of the threads of the box and pin are non interferingwith each other and at least some of the stab flanks of the threads ofthe box and pin interferingly engage each other when the connector isfully made up.
 11. A pipe connector, comprising: a box having aninternal threadform; a pin having an external threadform in a threadinterference engagement with the internal threadform of the box when theconnector is fully made up; and each of the threadforms having at leastsome threads differing in shape and engaging each other at differentpoints from other of the threads.
 12. The connector according to claim11, wherein the differing shapes create a greater amount of netcompressive load flank preload in a central portion than in upper andlower portions of the threadforms.
 13. The connector according to claim11, wherein the differing shapes create load paths that have anglesrelative to an axis of the connector that differ along the lengths ofthe threadforms.
 14. The connector according to claim 11, wherein: atleast some of the threads of the pin have load flanks that comprise acurved, convex outer portion and a curved, concave inner portion thatare joined to each other by a transition area; and a first line tangentto the transition area extends in an upward and inward direction to anintersection with an axis of the connector, a second line tangent to thetransition area extends in a downward and inward direction to anintersection with the axis of the connector, and a third line tangent tothe transition area extends in an upward and inward direction to anintersection with the axis of the connector.
 15. The connector accordingto claim 11, wherein: at least some of the threads of the box and pinhave load flanks that are non interfering with each other and at leastsome other of the threads of the box and pin have load flanks thatinterferingly engage each other when the connector is fully made up. 16.The connector according to claim 11, wherein: at least some of thethreads of the box and pin have stab flanks that are non interferingwith each other and at least some other of the threads of the box andpin have stab flanks that interferingly engage each other when theconnector is fully made up.
 17. A pipe connector, comprising: a boxhaving an internal threadform with a plurality of box threads separatedfrom each other by a root, the distance between each of the box threadsbeing at a selected pitch, each of the box threads having a load flankand a stab flank separated by a crest; a pin having an externalthreadform with a plurality of pin threads separated from each other bya root, each of the pin threads having a load flank and a stab flankseparated by a crest; the pin threads in a first portion of thethreadforms having crests of reduced height compared to the pin threadsin a second portion and a third portion of the threadforms; the loadflanks of the pin threads in the second and third portions having aconvex portion that joins the crest and a concave portion that joins theroot, the convex and concave portions being joined to each other by anS-shaped transition area; the load flanks of at least some of the pinthreads in the first portion having a concave portion joining the crestand lacking a convex portion and transition area, at least some of thebox threads in the third portion having crests of reduced heightcompared to the box threads in the first and second portions; the loadflanks of the box threads in the first and second portions having aconvex portion that joins the crest and a concave portion that joins theroot, the convex and concave portions being joined to each other by anS-shaped transition area; the load flanks of at least some of the boxthreads in the third portion having a concave portion joining the crestand lacking a convex portion and transition area; and wherein when theconnector is fully made up, in the first portion, convex portions of theload flanks of at least some of the box threads mate with concaveportions of the load flanks of at least some of the pin threads, in thesecond portion, convex and concave portions of the load flanks of atleast some of the box threads mate with concave and convex portions,respectively, of the load flanks of at least some of the pin threads,and in the third portion, concave portions of the load flanks of atleast some of the box threads mate with convex portions of the loadflanks of at least some of the pin threads.
 18. The connector accordingto claim 17, wherein the first portion is closer to a rim of the boxthan the second portion, and the second portion is closer to the rim ofthe box than the third portion.
 19. The connector according to claim 17,wherein when the connector is fully made up, the stab flanks of at leastsome of the pin and box threads in the first and second portions engageeach other, and at least some of the stab flanks of the pin and boxthreads spaced apart from each other by gaps.
 20. A pipe connector,comprising: a box having an internal threadform, an internal torqueshoulder spaced in a first direction from the internal threadform, andan annular recess extending in the first direction from the internaltorque shoulder; a pin having an external threadform threadingly engagedwith the internal threadform of the box and a pin torque shoulder spacedin the first direction from the external threadform, the external torqueshoulder engaging the internal torque shoulder when the connector isfully made up; and a pin seal member on the pin that extends in thefirst direction from the external torque shoulder and engages the recessin a metal-to-metal sealing engagement.
 21. The connector according toclaim 20, wherein the pin seal member has a free end that protrudes inthe first direction farther than any other portion of the pin.
 22. Theconnector according to claim 20, further comprising: a rim on the boxmember that is spaced in a second direction from the internalthreadform; a box seal member protruding from the rim in the seconddirection and sealing against an outer diameter portion of the pin in ametal-to-metal sealing engagement.
 23. The connector according to claim22, wherein the box seal member comprises: a convex seal rib joined byupper and lower concave recesses above and below the seal rib, the sealrib being spaced radially outward of an imaginary conical surface thatextends from a lower edge of the lower concave recess to an upper edgeof the upper concave recess.
 24. A pipe connector, comprising: a boxhaving an internal threadform; a pin having an external threadformthreadingly engaged with the internal threadform of the box, the pinhaving a metal-to-metal sealing surface on an outer diameter portion ofthe pin; a box seal member on the box, the box seal member having aconvex seal rib that seals in metal-to-metal engagement with the sealingsurface on the pin when the box and pin are made up; and the box sealmember having upper and lower concave recesses above and below the sealrib, the seal rib being spaced radially outward of an imaginary conicalsurface that extends from a lower edge of the lower concave recess to anupper edge of the upper concave recess.
 25. The pipe connector accordingto claim 24, wherein the seal rib has a minimum inner diameter that islarger than a diameter of the imaginary conical surface at a point onthe imaginary conical surface closest to the seal rib.
 26. The pipeconnector according to claim 24, wherein the box seal member has a freeupper edge that defines an uppermost point on the box.
 27. A pipeconnector, comprising: a box having an internal threadform and aninternal torque shoulder spaced in a first direction from the internalthreadform; a pin having an external threadform threadingly engaged withthe internal threadform of the box and an external torque shoulderspaced in the first direction from the external threadform, the externaltorque shoulder engaging the internal torque shoulder when the connectoris fully made up; and the box having a rim spaced in a second directionfrom the internal threadform, the rim comprising a box seal member thatengages an outer diameter portion of the pin in a metal-to-metal sealingengagement.
 28. The connector according to claim 27, wherein: the boxhas an external shoulder of selected wall thickness and spaced in thesecond direction from the internal threadform; and the box seal memberextends from the external shoulder in the second direction and haslesser wall thickness than the box at an outer diameter of the externalshoulder.
 29. The connector according to claim 27, wherein the box sealmember has a free end that protrudes in the second direction fartherthan any other portion of the box.
 30. The connector according to claim27, wherein the box seal member comprises: a convex seal rib joined byupper and lower concave recesses above and below the seal rib, the sealrib being spaced radially outward of an imaginary conical surface thatextends from a lower edge of the lower concave recess to an upper edgeof the upper concave recess.
 31. A pipe connector, comprising: a boxhaving an internal threadform; a pin having an external threadformthreadingly engaged with the internal threadform of the box; and the pinhas threads cut on an axial end thereof, and is upset at the axial endincluding an axial upset length “L” and a radial upset dimension “e”,such that an aspect ratio L/e≧30.
 32. The connector according to claim31, wherein the threadform has an axial length that is substantially nomore than two thirds the axial upset length L.